The invention relates to an electric motor having a rotor and a stator as well as a measuring device designed for detecting a position of the rotor, which is movable relative to the stator, and comprising a sensor system for the provision of sensor signals in dependence on the position of the rotor relative to the stator and a processing device for controlling the sensor system, for processing the sensor signals and for outputting a position signal. The invention further relates to a method for the operation of an electric motor.
From EP 1 471 331 A2, a method for the detection of the position of a moving element by means of a sensor is known, which method involves a magnetisation at the element. If the operating voltage is switched on, the position is detected by a magnet-sensitive sensor as a counting result of an alternating north pole/south pole magnetisation; if the operating voltage is switched off, the sensor is supplied with energy from an energy accumulator, and the counted poles are stored in a memory device.
EP 0 995 974 A1 discloses a rotary encoder with an encoder shaft supporting a first code disc for single-turn measurement and a second code disc for multi-turn measurement. The rotary encoder comprises a logic module containing signals of evaluation electronics assigned to the multi-turn code disc. This logic module is only supplied with current/voltage at a level change of the signals of a sensor or at a timing pulse.
Such methods are, among other purposes, used to determine the position of a rotor relative to a stator of an electric motor, such as an electronically commutated electric motor, in order to supply the coils of the electric motor with power as efficiently as possible. In a permanent supply of an electric motor with electric energy, the position of the rotor relative to the stator is determined at the time of start-up and then pursued further by means of an incremental position sensor. An (emergency) stop of such an electric motor, however, involves a need for a renewed determination of position. In order to avoid this initialisation of the electric motor after each stop, known methods and devices rely on position detection based on an internal energy accumulator, such as a battery, if the external voltage supply, for example a mains voltage for the motor, fails.